Images using both a visible dye and a fluorescent dye have hitherto been used as images that have a security measure such as for reproduction prevention purposes. An example of a conventional method for preparing this print is to form an image, by thermal diffusion transfer of visible dyes such as yellow dyes, magenta dyes, and cyan dyes, on which image a latent image is then formed by thermal ink transfer or thermal diffusion transfer of a fluorescent dye that emits fluorescence upon exposure to ultraviolet light.
When a latent image is provided by thermal ink transfer, however, concave/convex is present in the image although it is colorless. The concave/convex is visible under visible light without the application of ultraviolet light. Therefore, the image could not be said to be a completely latent image. Further, also when a protective layer is provided so as to cover the surface of the latent image, in some cases, the concave/convex of the image is visible under visible light, and it was difficult to form a completely latent image.
FIG. 1 is a diagram illustrating a conventional technique using such thermal ink transfer. When a fluorescent dye 15 is transferred by thermal ink transfer onto image receiving paper 11, onto which a yellow dye 12, a magenta dye 13, and a cyan dye 14 have been transferred, the fluorescent dye 15 part is raised. Therefore, due to the presence of a concave/convex part, even when a protective layer 16 is provided, a completely invisible image cannot be provided.
On the other hand, when a latent image is provided by thermal diffusion transfer, the problem of concave/convex of the image can be solved. In this case, however, upon exposure of the visible dyes to heat at the time of the transfer of the fluorescent dye, the visible dyes are disadvantageously transferred onto the fluorescent dye ink sheet (hereinafter referred to as “backtrap”). As a result, color of the visible dye image on its part where the latent image has been formed sometimes becomes partly light. Thus, when the color of the visible dye has become light, the pattern of the latent image is disadvantageously visible. Further, when the visible dyes and the fluorescent dye are present together, energy transfer between dyes occurs, leading to a problem of a lowering in or complete loss of fluorescence intensity of the fluorescent dye.
FIG. 2 is a schematic diagram illustrating the above conventional technique using thermal diffusion transfer. A yellow dye, a magenta dye, and a cyan dye as visible dyes are thermally transferred in that order on image receiving paper 21, and a fluorescent dye, is thermally transferred thereon. In this case, upon exposure to heat at the time of the thermal transfer of the fluorescent dye, a part of the visible dyes (22 to 24) is transferred onto a fluorescent dye ink ribbon 27 (a transferred part being indicated by 28). Further, when the visible dyes and the fluorescent dye are thermally transferred in this order, disadvantageously, the luminescence intensity of the fluorescent dye is extremely lowered. Although the reason for this has not been fully elucidated yet, it is believed that, since the fluorescent dye is transferred onto the layer in which the visible dyes such as the yellow dye, the magenta dye, and the cyan dye have already been diffused, substantially the entire part of the transferred fluorescent dye interacts with the visible dyes, whereby the fluorescence of the fluorescent dye disadvantageously becomes extinct. In FIG. 2, numeral 22 designates a region where the yellow dye is mainly present, numeral 23 a region where the yellow dye and the magenta dye are mainly present, numeral 24 a region where the yellow dye, the magenta dye, and the cyan dye are mainly present, and numeral 25 a region where the yellow dye, the magenta dye, the cyan dye, and the fluorescent dye are mainly present.